This invention relates generally to out-of-step (power swing) conditions on a power line, and more specifically concerns the detection of an out-of-step condition following clearance of an external fault on the line.
In many power systems, particularly those in less developed countries, a condition known as power swing can occur, caused by various system conditions. A power swing can result in severe system disturbances, and is generally known in the art as an out-of-step condition. Basically, for a typical power system, during normal operation, the output of electric power from an electric power generator will produce an electric torque which balances the mechanical torque applied to the rotor shaft of the generator. Ideally, the electric power generator rotor runs at a constant speed, because of the balance of electric and mechanical torques. When a fault on the power system occurs, reducing the amount of power transmission from the generator, the electric torque, which normally balances the mechanical torque, will also decrease. If the mechanical power is not reduced during the time of the fault, the generator rotor will accelerate.
Referring now to FIG. 1, after a fault occurs, with the power output being reduced to PF from P0, the generator rotor swill start to accelerate, and the angle xcex4 between the two source generators on the line (power P is transferred between the two generators in operation of the power system) will start to increase. At the time that the fault is cleared, when the angle difference reaches xcex4c, there is a decelerating torque acting on the rotor because the electric power PC at the angle xcex4C is larger than the mechanical power input P0. However, because of the inertia of the rotor system of the motor, the angle xcex4 will not start to go back to xcex40 immediately, but rather continue to increase to xcex4F, when the energy lost during deceleration in area 10 of the power angle curve of FIG. 1 is equal to the energy gained during the acceleration in area 12. If xcex4P is smaller than xcex4L, then the system is transiently stable.
With sufficient damping, the angle difference xcex4 of the two sources eventually goes back to the original balance point of xcex40. However, in the situation where area 10 is smaller than the area 12 at the time that the angle reaches xcex4L, then a further increase in the angle will result in an electric power output that is smaller than a mechanical power output, such that the generator rotor will accelerate again and xcex4 will increase beyond the point of typical operational recovery, resulting in a transiently unstable situation, which is shown in the curve of FIG. 2. When such an unstable situation of the power system occurs, one equivalent generator rotates at a speed that is different than the other equivalent generator, which is a classic out-of-step (OOS) condition.
An out-of-step power system condition, besides providing inherent stability problems for the system, also may be evaluated by certain distance and phase overcurrent elements in the protective relay as a fault as opposed to an out-of-step condition. The protective relay will then operate to trip circuit breakers associated with the relay in response to the out-of-step condition, adding to the instability of the system. Such a response of the relay is thus undesirable.
Traditionally, such as described in U.S. Pat. No. 5,731,943, the rate of change of the positive sequence impedance (Z1) is monitored to detect an out-of-step condition; the operation of the distance protection elements are blocked if the impedance rate of change indicates an out-of-step condition rather than a fault. The positive sequence impedance measurement is used because the change of that impedance into a protection region defined by the protection elements of the protective relay is a slow process during an out-of-step condition, while the impedance moves rapidly from a load region into a protection region in the impedance plane when an actual fault occurs.
FIGS. 3A and 3B show illustrative double binder impedance characteristics in the impedance plane used to detect an out-of-step condition and provide a blocking signal in the distance elements. In these examples, (FIG. 3A is exemplary), the outer protection boundary impedance element xcex4 is located inside the load region 22. While an inner protection boundary impedance element is placed outside of the over-reaching zone 2 boundary.
Typically, in order to prevent distant elements in the protective relay from operating in response to an out-of-step condition, it is conventional to block the instantaneous zone 1 distance element and the forward direction overreaching zone 2 element used in a communication/assisted tripping scheme. The inner protection zone boundary impedance element must thus be located outside of the overreaching zone 2 region. Under certain conditions at the time of fault clearance, the positive sequence Z1 impedance measured by a distance relay may already be in a protection region. If after fault clearance occurs, the impedance does not stay between the inner and outer impedance measurement elements in the impedance plane, the conventional out-of-step logic will fail to operate and will not block the distance element of operation. This will aggravate the disturbance of a power system in an out-of-step condition.
Hence, while traditional double blinder out-of-step blocking (OSB) arrangements operate correctly under most circumstances to block the operation of distance elements during an out-of-step condition, an external multi-phase/three-phase fault which is slowly cleared may result in a failure to pick up the out-of-step blocking condition because the Z1 impedance is already within the inner impedance region of out-of-step detection logic and the impedance may not stay between the inner and outer impedance measurement elements. Hence, there is a need to be able to detect an out-of-step condition for the system condition following clearance of external faults.
Accordingly the present invention is an apparatus for use in a protective relay for detecting an out-of-step condition following clearing of an external multi-phase fault, on a power line, comprising: a circuit for detecting the presence of a multi-phase fault on the power line; a timing circuit for readying, i.e. xe2x80x9carmingxe2x80x9d, an out-of-step logic circuit if the multi-phase external fault remains for a preselected period of time; means for determining the positive sequence impedance on the power line; and a circuit for declaring an out-of-step condition and for blocking selected distance elements of the protective relay if the positive sequence impedance remains inside a selected impedance plane boundary of protection for a qualified period of time following clearing of the multi-phase external fault.